Transcription of Subthreshold Operation and gm/Id design - CppSim
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Subthreshold Operation and gm/Id design - CppSim
PerrottAnalysis and design of Analog Integrated CircuitsLecture 16 Subthreshold Operation and gm/IdDesignMichael H. PerrottApril 1, 2012 Copyright 2012 by Michael H. PerrottAll rights PerrottA Closer Look at Transconductance Assuming device is in strong inversion and in saturation:2 IdVgsId_op VVTHVgs_opVds > VM1 IdVgsNMOS gsdgm = Vgs IdVgs_opID= nCox2WL(Vgs VTH)2(1 + Vds) gm= Id Vgs nCoxWL(Vgs VTH) s2 nCoxWLId gm Idq2 nCoxW/L Id 2Id(Vgs VTH) PerrottUnity Gain Frequency for Current Gain, ft Under fairly general conditions, we calculate: ftis a key parameter for characterizing the achievable gain bandwidth product with circuits that use the device3|Id/Iin| PerrottCurrent Density as a Key Parameter Current density is defined as the ratio Id/W:-We ll assume that current density is altered by keeping Idfixed such that only W varies Maintains constant power ro(.)
M.H. Perrott Comparison of Strong and Weak Inversion for g m Assumption: I d is constant with only W varying Strong inversion formulation predicts ever increasing g m with reduced overdrive voltage-Reduced current density leads to reduced overdrive voltage and therefore higher g m Weak inversion formulation predicts that g m will hit a maximum value as current density is reduced
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